The present invention relates in general to a method of forming a protective coating on a substrate by means of plasma polymerization with at least one gaseous silicon-organic compound, in which a plasma is generated in a vacuum chamber and a gas with the compound is fed thereto for the production of the coating. The invention relates further to a coating apparatus with a coating chamber, a source arrangement which, for the generation of a plasma, generates an electromagnetic alternating field, a pumping arrangement connected to the chamber, and a controlled gas inlet arrangement for the infeed of gas into the chamber.
In the technology of protective coatings it is generally known to provide substrates, specifically non-planar substrates such as reflectors for automobile or other headlights, floodlights, etc. with corrosion proof or wiping resistant, protective coatings.
Plasma polymerization which, as is generally known, is used for this task, and which is a plasma enhanced chemical deposition method operative at comparatively low substrate temperatures, permits metal substrates or previously coated metal layers, specifically aluminum vapor coated substrates, to be coated with siloxane, a silicon-organic compound.
Because a plasma is generated for this procedure by direct current, as disclosed, for example, in the German patent publication DE-OS 22 63 480 or French patent reference FR A 21 690 072, a new metal encasement must be furnished from time to time, around at least one of the metal electrodes used for the coating process and between which the plasma is generated after every coating process. This is because an insulating coating is produced at relatively high coating rates on the electrode during coating with siloxane. Without this new metal encasement, the longer the process continues, the more dielectric coating is formed on the electrode. This would lead to an unstable plasma with flashovers through the insulating coating and contamination of the coating process area, sputtering-off until, finally, a disruption or break down of the plasma discharge takes place.
This problem remains if, according to German reference DD A 272 773, a pulsed DC-plasma is used. According to DD A 272 773, an alternating voltage of selected frequency is rectified in one direction and is used for the plasma generator. By means of a superimposed DC-voltage, the total voltage is raised causing the plasma discharge to intensify which, it is said, increases productivity. It is also stated in the reference that an upper limit for this DC-voltage exists, above which the discharge becomes continuous. This pulsed switching of a discharge with a non-pulsed operating voltage (a rectified voltage) can no longer support pulsed DC-discharge operation. A pulsed DC-discharge is used according to DD A 272 773 for preventing arcing, which, as is known, is often encountered in pure DC-discharge operation.
Due to the necessity of having to reencapsulate the mentioned electrode, relatively long maintenance intervals occur due to which a production plant must often shut down for prolonged time spans. This can not be tolerated with in-line production plants, for example.
It is noted that the parameters for a plasma polymerization method depend very strongly on the gaseous compounds used and which will thus form the process coating. It is known from European patent reference EP A 207 767, to pulsate RF plasma, during a surface treatment-etching or coating by plasma enhanced reactive processes. Without any specific selection, a large number of different materials are proposed for processing in this reference, e.g., Si.sub.3 N.sub.4, TiO.sub.2, Al.sub.2 O.sub.3, BN, SiO.sub.2, B.sub.4 C, SiC, HC, TiC, TiN, BP. All of these coating materials are not produced by polymerization.
In case of the plasma polymerization of silicon-organic compounds, it is known, e.g. from EP A 0 299 754 to generate the plasma by means of an electormagnectic alternating field. It is further known that, when using certain non-silicon organic gaseous compounds such as methane, higher deposition rates can be obtained by pulsing the electromagnetic alternating field, and at the same time applying average power.
How the coating process behaves, as noted above, depends on plasma modulation, and to a large extend, on the specific gaseous compound supplied for the coating operation.
Regarding the recognition that it is possible to reach, in certain cases, higher coating rates by a pulsed electromagnetic alternating field for the generation of the plasma at the same average electrical power applied also under the same process conditions, reference is made to:
Vinzant J, Shen M, Bell A, ACS SYMP.SER; SHEN BELL 108 p. 79 (1979), "Polymerization of Hydrocarbons in a pulsed plasma";
H. Yasuda, T. Hsu: J. Polym Sci, Polym Chem ed 15, 81, (1977), or
H. Yasuda "Plasma polymerization" Academic press (1985), p.103-105; and
Lloret A, Bertran E, Andujar J, Canillas A, Morenza J, J.APPL.PHYS. 69, p. 632 (1991), "Ellipsometric study of a-Si:H thin films deposited by square wave modulated rf glow discharge".
Further, that a lower heat loading of the substrate takes place by the application of a pulsed electromagnetic alternating field for the generation of the plasma follows from:
GB A 2 105 729 (1981), ITT Industries Ltd., London-GB; and
U.S. Pat. No. 4,950,956 (1990), Anelva Corp., Tokyo, Japan.
It is furthermore, also known that new coating properties are realized by the application of a pulsed electromagnetic alternating field for the generation of a plasma. Reference is made in this respect to:
JP 62 103 371 (1985), Hitachi K K;
DD 264 344 A3 (1986), VEB ZFT Mikroelectronik.
Improvement in the purity of the layer or a reduction of powdery layer portions is known:
in relation to the depositing of diamond or graphite out of methane, from JP 62 123 096 (1985), Showa Denko K K;
for silane (SiH.sub.4) from: Watanabe Y, Shiratani M, Kubo Y, Ogawa I, Ogi S, APPL.PHYS.LETT. 53, - p. 1263 (1988), "Effects of low frequency modulation on rf-discharge chemical vapor deposition";
with reference to the deposition of amorphous carbon from methane, from Hossary F, Fabian D, Webb A, THIN SOL.FILM 192, p.201 (1990), "Optical properties of amorphous carbon films formed by rf-plasma deposition from methane"; and
again with reference to silane (SiH.sub.4) further reference is made to: Watanabe Y, Shiratani M, Makino H, APPL.PHYS.LETT. 57, p. 1616 (1990), "Powder-free plasma chemical vapor deposition of hydrogenated amorphous silicon with high rf power density using modulated rf discharge."
All of the above documents are to be taken as an integrated part of the present description and are to be considered as incorporated in the present description by reference.